The regulation of skeletal muscle development by the canonical muscle regulatory factors and associated regulatory proteins has been intensively investigated. The role of homeobox proteins in muscle development, however, remains less well understood. We showed previously that the homeobox transcription factor Barx2 promotes myoblast differentiation and regulates muscle specific gene expression in vitro. Our preliminary in vivo studies show that Barx2 is expressed in embryonic myoblasts and satellite cells and suggest that the Barx2 null mouse has defects in muscle development. Our proposed studies focus on defining the roles of Barx2 in embryonic muscle development (Aim 1) and in adult muscle function and repair (Aim 2), and on identifying the molecular targets that may mediate these activities (Aim 3). To define the influence of Barx2 on embryonic muscle development, we will determine whether it is co- expressed with markers of migrating, proliferating and differentiating myoblasts, test the proliferation and differentiation potential of Barx2 null myoblasts, and examine muscle masses in embryonic and perinatal Barx2 null mouse limbs. The function of Barx2 in adult muscle will be examined by analyzing the ratio of fast to slow fiber types and measuring contractile muscle force in Barx2 null mice. Models of both acute (cardiotoxin-induced injury) and chronic (mdx dystrophic mice) muscle injury will be used to examine the role of Barx2 in satellite cell-mediated muscle repair. To identify the targets of Barx2 that may control muscle development, we will perform microarray analysis of mRNA from Barx2 null and wild type muscle and use a new chromatin immunoprecipitation (ChlP)-cloning methodology that we have developed to discover genomic binding sites for Barx2. Defining the roles of Barx2 in muscle development, function and repair should help to build a more complete understanding of the muscle regulatory network that may aid in diagnosing or treating muscle disease.